Magyar R J, Tretiak S
NIST Center for Theoretical and Computational Nanosciences (NCTCN), Gaithersburg, Maryland 20899, and Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545.
Theoretical Division and Center for Nonlinear Studies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545.
J Chem Theory Comput. 2007 May;3(3):976-87. doi: 10.1021/ct600282k.
Time-dependent density functional theory (TDDFT) is a powerful tool allowing for accurate description of excited states in many nanoscale molecular systems; however, its application to large molecules may be plagued with difficulties that are not immediately obvious from previous experiences of applying TDDFT to small molecules. In TDDFT, the appearance of spurious charge-transfer states below the first optical excited state is shown to have significant effects on the predicted absorption and emission spectra of several donor-acceptor substituted molecules. The same problem affects the predictions of electronic spectra of molecular aggregates formed from weakly interacting chromophores. For selected benchmark cases, we show that today's popular density functionals, such as purely local (Local Density Approximation, LDA) and semilocal (Generalized Gradient Approximation, GGA) models, are qualitatively wrong. Nonlocal hybrid approximations including both semiempirical (B3LYP) and ab initio (PBE1PBE) containing a small fraction (20-25%) of Fock-like orbital exchange are also susceptible to such problems. Functionals that contain a larger fraction (50%) of orbital exchange like the early hybrid (BHandHLYP) are shown to exhibit far fewer spurious charge-transfer (CT) states at the expense of accuracy. Based on the trends observed in this study and our previous experience we formulate several practical approaches to overcome these difficulties providing a reliable description of electronic excitations in nanosystems.
含时密度泛函理论（TDDFT）是一种强大的工具，可用于精确描述许多纳米级分子系统中的激发态；然而，将其应用于大分子时可能会遇到一些困难，这些困难从之前将TDDFT应用于小分子的经验中并非立即显而易见。在TDDFT中，已表明在第一光学激发态以下出现的虚假电荷转移态对几种供体-受体取代分子的预测吸收和发射光谱有显著影响。同样的问题也影响了由弱相互作用发色团形成的分子聚集体的电子光谱预测。对于选定的基准案例，我们表明，如今流行的密度泛函，如纯局域（局域密度近似，LDA）和半局域（广义梯度近似，GGA）模型，在定性上是错误的。包括半经验（B3LYP）和从头算（PBE1PBE）且含有一小部分（20 - 25%）类福克轨道交换的非局域杂化近似也容易出现此类问题。像早期杂化泛函（BHandHLYP）那样含有较大比例（50%）轨道交换的泛函，虽以准确性为代价，但显示出虚假电荷转移（CT）态要少得多。基于本研究中观察到的趋势以及我们之前的经验，我们制定了几种实用方法来克服这些困难，从而对纳米系统中的电子激发提供可靠描述。
